1. Field of the Invention
The invention relates to equipment and processes for generating useful power. More particularly, the invention provides a combined gas and steam turbine cycle, wherein a split stream heat recovery boiler is applied to recover heat from a gas turbine exhaust flow.
2. Description of the Prior Art
Aero engines with relatively low exhaust temperatures of from 750.degree. F. to 900.degree. F. do not achieve steam throttle temperatures high enough for the next generation of high-temperature-topping steam turbines. The latest "F" and future "G" technology heavy-duty gas turbines have exhaust temperatures of from 1100.degree. F. to 1160.degree. F. that also are too low for steam throttle temperatures of from 1400.degree. F. to 1500.degree. F. Currently used full exhaust gas stream supplementary firing in front of the boiler tends to lower combined cycle efficiency and thus nullifies potential efficiency gains.
Research and development has been conducted in Europe on higher throttle temperatures and supercritical pressures (boilers and steam turbines) for realizing more efficient conventional steam power plants. In the United States, the Department of Energy (DOE) and the Electric Power Research Institute (EPRI) through the Innovative Steam Technologies/Solar Turbine Inc. (IST/Solar) are pursuing the development of high temperature and high pressure (HT/HP) topping steam turbines (TST) for existing and future conventional all-steam power plants. Parallel work is also being done on the once through boiler (OTB).
The "F" technology gas turbine introduced in the early 1990s fires at 2300.degree. F. and has made the reheat (RH) heat recovery steam generator (HRSG) a reality. Higher combined cycle (CC) efficiency is achieved with this technology. The promise of the "G" turbine that is projected to fire at about 2600.degree. F. offers even higher CC efficiency. A typical 160 MW "F" gas turbine exhausting into a RH HRSG produces a CC efficiency of about 53% (LVH), and the "G" turbine is projected to have a CC efficiency of about 56%. There is a distinct probability that future CC efficiency may exceed 60% by utilizing the concepts of this invention. Supplementary firing the entire exhaust gas stream greatly increases the power output of the overall bottoming portion of the cycle. The very bottom end has a low heat-to-power efficiency conversion level that at most is only 30%. Studies have shown that the higher throttle temperature does not yield a higher overall combined cycle efficiency for reasons to be explained. Therefore, the HT/HP topping steam turbine has not been given serious consideration for combined cycles for either the Aero or heavy duty gas turbines. Conventional CCs generally do not incorporate supplementary firing. However, the conventional all-steam power plants do show a cycle efficiency gain when the HT/HP topping unit is utilized.
The Aero engines with high cycle pressure ratios have low exhaust temperatures by cycle inheritance low exhaust temperatures. Even the new proposed intercooled engines with TITs of 2600.degree. F. and 40 pressure ratios are projected to have exhaust temperature levels of only 850.degree. F. Full exhaust flow supplementary firing does not render markedly increased combined cycle efficiency even when applying a higher temperature topping steam turbine. The steam-injected (STIG) gas turbines have lower exhaust temperatures of about 750.degree. F., and the regenerative cycle gas turbines have even lower exit temperatures (650.degree. F. level). The addition of a HT/HP TST appears impractical. Supplementary firing in front of the HRSG does not offer a viable solution towards higher cycle efficiency.
Supplementary firing the exhaust gas of gas turbines in front of HRSGs has been done for years to increase HRSG output. However, such firing tends to lower CC efficiency. More power is generated but at a lower overall CC efficiency due to the increasing dominance of the lower efficiency of the bottoming part of the cycle. Foster-Pegg in 1982 reported that extensive Westinghouse Electric Corp. studies showed that maximum CC efficiency takes place at a point where an optimum number of drum pressure levels are used--i.e. three stages--to obtain a minimum stack temperature and where no supplementary firing is introduced. General Electric Co. (GE) studies revealed the same peaking phenomena as reported by Tomlinson and Lee in 1985. Because of this optimization point, and since a reduction in CC efficiency occurs with supplementary firing, CC power plants that do not export steam for industrial use do not apply supplementary firing.
Early conceptual work on the chemical recuperated gas turbine CC burning natural gas fuel was performed by Jack Janes of the California Energy Commission (CEC). General Electric Co., under contract to the CEC, made a study of this cycle. In turn, GE was granted a U.S. patent on a portion of the cycle (U.S. Pat. No. 5,133,180, issued Jul. 28, 1992). In this patent, the steam-injected gas turbine CC is used so that the gas turbine exhausts into a combination HRSG and hydrocarbon reformer. A full exhaust flow duct burner (supplementary firing) is positioned in front of the reformer. An excess amount of low efficiency steam is generated in the process, which degrades CC efficiency.
Basic reforming theory is outlined in the '180 patent. The fuel gas as a mixture of natural gas and steam is heated in the presence of a catalyst so that endothermic reaction occurs. The extent of natural gas reforming to produce hydrogen and CO is a function of pressure, temperature and steam/fuel ratio. The latter may be controlled by steam flow to and from the two boiler streams. The proper steam to natural gas ratio for reforming is generally considered to range from 3 to 5 moles of steam to one mole of natural gas. Therefore, 25 to 35% of the total steam produced in a conventional non-supplementary fired gas turbine heat recovery boiler may be used for natural gas fuel reforming. A higher pressure tends to reduce hydrogen production, but a higher temperature will appreciably increase hydrogen production. A reforming temperature of 1300.degree. to 1500.degree. F. will produce far more hydrogen than a 1100.degree. F. temperature, as is well known in the hydrocarbon process industry. Natural gas reformers generally use these higher temperatures. The key to reforming is the catalyst and the temperature. New catalysts could improve reforming at lower temperatures, but so far such catalysts have not been developed.
The hydrogen content is important for reducing the NOx and minimizing the CO, indicative of incomplete combustion. The hydrogen component increases the flame travel speed for improved combustion for low BTU/Ft.sup.3 gaseous fuels burning at low flame temperatures. NOx formation is directly proportional to flame temperature. A low BTU/Ft.sup.3 value of a mixture of reformed natural gas and steam will bum at a low flame temperature. Very little NOx will be formed, and the CO will also be very low. Combustion tests sponsored by DOE and EPRI have verified this phenomenon. Hydrogen-rich fuel exhibits high flame velocity, combustion stability, wide flammability limits and low luminosity and thus is well suited as a gas turbine fuel.
The '180 patent suggests that the reheat gas turbine is too complicated and costly to consider. Asea Brown Boveri (ABB) has introduced an all new family of reheat gas turbines, the GT-24 and 26 models (60 Hertz 165 and 50 Hertz 230 MWs, respectively). This introduction appears to neutralize the concerns expressed in the '180 patent about complication and cost. These new reheat gas turbines will have a 30 pressure ratio and will exhaust at about 1150.degree. F., which is still too low a temperature for reforming natural gas with a resulting adequate hydrogen content for low NOx and CO combustion. Therefore, the present invention may be advantageously applied to these new reheat gas turbines.
With the appearance of an increasing number of high temperature and high-pressure-ratio gas turbines, steam-injected gas turbines and other types of gas turbines with low and moderate exhaust temperatures; with the development of high temperature and high pressure topping steam turbines and the once through boiler; and with the prospects of developing chemical recuperation equipment for gas turbines, the use of such equipment according to the process and teachings of the present invention becomes technically feasible. The present invention has practical utility for the generation of useful power, such as electrical power, for a number of different gas turbine, boiler and steam turbine arrangements.